1. Field of the Invention
The present invention relates to cavitation in propellers and, more particularly, to an arrangement for providing air to a propeller for reducing cavitation.
2. Description of the Prior Art
Cavitation has many deleterious effects on the operation of a ship and the components associated with propulsion thereof. Especially in warships, which operate at high speeds which may produce cavitation, the effects of cavitation are especially deleterious. No only does cavitation reduce performance of a warship, but since warships preferably should move through the water with stealth, as quietly as possible in order to avoid detection by listening devices, there has been a great need for a reduction and minimization of cavitation in ships and especially warships.
Cavitation caused by the rotation of a fixed or a controllable-pitch propeller generates noise which can easily be detected by modern, sophisticated devices available for military use today. To reduce the propagation of noise caused by cavitation, it is known that air can be fed into the vicinity of the leading edges of the propeller blades or fins. The air is released therefrom to aid in the dissipation and minimization of the noise created by the cavitation, or to prevent a potentially cavitating propeller from cavitating. Such an arrangement is described in the German Patent Publication Laid Open for Opposition Purposes No. DE-OS 30 05 680 which is incorporated herein by reference as if the full text thereof is set forth herein. Another example of a cavitating propeller having air ejection is found in U.S. Pat. No. 4,188,906 which is entitled "Supercavitating Propeller with Air Ventilation" which is incorporated herein by reference as if the full text thereof is set forth herein. In this U.S. patent, a method of decreasing the deleterious effects of cavitation on a ship propeller is accomplished by ejecting air from the suction side of each propeller blade when the ship speed and propeller speed reach a predetermined point which thereby could produce a cavity which extends from the leading edge to a point in the water beyond the trailing edge of the propeller and may envelope the entire section of the blade.
Hitherto, the retrofitting of a propeller with air injection to reduce cavitation has included a very high expense of fitting the ship with such an air feed installation. In addition, the maintenance of prior art air ejection systems has been very high and therefore, not practical especially in the case of a war vessel.
Controllable pitch propellers are known in the prior art, some examples of which are U.S. Pat. No. 4,474,533 entitled "Controllable Pitch Propeller Assembly", U.S. Pat. No. 4,436,482 entitled "Constant Ship Speed Control Method" which discloses a method of controlling the speed of a ship equipped with a controllable pitch propeller. Another example is disclosed in U.S. Pat. No. 4,142,829 entitled "Compound Remote Control Device for the Propulsion Engine of a Ship's VariablePitch Propeller". Yet another example is disclosed in U.S. Pat. No. 4,150,921 entitled "Built-Up Marine Propellers with Adjustable Pitch and Axial Removal Blades". All of the above U.S. patents are incorporated herein by reference.
Typically, ships have no room left for the addition of any equipment to the propeller or other portions thereof associated therewith after the time of building. Therefore, retrofitting is an extremely difficult problem which requires special equipment which heretofore has not been generally available.